Relational databases allow defining constraints on data, such as uniqueness check, referential integrity check or value domain checks. These constraints need to be checked during insertion or update of the data in the database. Since database tables are inherently shared structures, it's not easy to parallelize constraint checking inter-query, except simple value domain checks, which are out-of-scope for this application.
Checking constraints normally doesn't pose any serious problem in “normal” databases, where the execution time is by far dominated by I/O time. However, with in-memory databases, the situation is different. Since no disk I/O is involved and CPU power is abundant, lack of inter-query parallelization of constraint checking can seriously hamper performance.